diff --git a/config/hcl2shim/flatmap.go b/config/hcl2shim/flatmap.go
index 2f7954d76..bcecb30df 100644
--- a/config/hcl2shim/flatmap.go
+++ b/config/hcl2shim/flatmap.go
@@ -356,6 +356,11 @@ func hcl2ValueFromFlatmapSet(m map[string]string, prefix string, ty cty.Type) (c
 		return cty.UnknownVal(ty), nil
 	}
 
+	// Keep track of keys we've seen, se we don't add the same set value
+	// multiple times. The cty.Set will normally de-duplicate values, but we may
+	// have unknown values that would not show as equivalent.
+	seen := map[string]bool{}
+
 	for fullKey := range m {
 		if !strings.HasPrefix(fullKey, prefix) {
 			continue
@@ -370,6 +375,12 @@ func hcl2ValueFromFlatmapSet(m map[string]string, prefix string, ty cty.Type) (c
 			key = fullKey[:dot+len(prefix)]
 		}
 
+		if seen[key] {
+			continue
+		}
+
+		seen[key] = true
+
 		// The flatmap format doesn't allow us to distinguish between keys
 		// that contain periods and nested objects, so by convention a
 		// map is only ever of primitive type in flatmap, and we just assume
@@ -386,5 +397,6 @@ func hcl2ValueFromFlatmapSet(m map[string]string, prefix string, ty cty.Type) (c
 	if len(vals) == 0 {
 		return cty.SetValEmpty(ety), nil
 	}
+
 	return cty.SetVal(vals), nil
 }
diff --git a/config/hcl2shim/flatmap_test.go b/config/hcl2shim/flatmap_test.go
index 56c06c3dc..07f93ac26 100644
--- a/config/hcl2shim/flatmap_test.go
+++ b/config/hcl2shim/flatmap_test.go
@@ -635,6 +635,50 @@ func TestHCL2ValueFromFlatmap(t *testing.T) {
 				}),
 			}),
 		},
+		{
+			Flatmap: map[string]string{
+				"single.#":                 "1",
+				"single.~1.value":          "a",
+				"single.~1.optional":       UnknownVariableValue,
+				"two.#":                    "2",
+				"two.~2381914684.value":    "a",
+				"two.~2381914684.optional": UnknownVariableValue,
+				"two.~2798940671.value":    "b",
+				"two.~2798940671.optional": UnknownVariableValue,
+			},
+			Type: cty.Object(map[string]cty.Type{
+				"single": cty.Set(
+					cty.Object(map[string]cty.Type{
+						"value":    cty.String,
+						"optional": cty.String,
+					}),
+				),
+				"two": cty.Set(
+					cty.Object(map[string]cty.Type{
+						"optional": cty.String,
+						"value":    cty.String,
+					}),
+				),
+			}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"single": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"value":    cty.StringVal("a"),
+						"optional": cty.UnknownVal(cty.String),
+					}),
+				}),
+				"two": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"value":    cty.StringVal("a"),
+						"optional": cty.UnknownVal(cty.String),
+					}),
+					cty.ObjectVal(map[string]cty.Value{
+						"value":    cty.StringVal("b"),
+						"optional": cty.UnknownVal(cty.String),
+					}),
+				}),
+			}),
+		},
 	}
 
 	for _, test := range tests {